Electro-optical displays and mesogenic light modulation media are known, which are in the isotropic phase when being operated in the display (hereinafter referred to as “isotropic mode displays”). Such displays and media are for example described in DE 102 17 273 A. Furthermore, electro-optical displays and mesogenic light modulation media are known, which are in the optically isotropic blue phase, when being operated in the display (hereinafter referred to as “blue mode displays”). Such displays and media are for example described in WO 2004/046 805.
Blue mode and isotropic mode displays provide several significant advantages compared to well-known and widely used displays using liquid crystals in the nematic phase, like for example liquid crystal displays (LCDs) operating in the twisted nematic (TN)-, the super twisted nematic (STN)-, the electrically controlled birefringence (ECB)-mode with its various modifications and the in-plane switching (IPS)-mode. Amongst these advantages are most pronounced their much faster switching times, and significantly wider optical viewing angle.
Whereas, compared to displays using mesogenic media in another liquid crystalline phase, as e.g. in the smectic phase in surface stabilized ferroelectric liquid crystal displays (SSF LCDs), the blue mode and isotropic mode displays of DE 102 17 273 A and WO 2004/046 805 are much easier to manufacture. For example, they do not require a very thin cell gap and in addition the electro-optical effect is not very sensitive to small variations of the cell gap.
However, the liquid crystal media that are commonly used in these blue mode and isotropic mode displays mentioned still require operating voltages, which are not low enough for some applications. Further the operating voltages of these media vary with temperature, and it is generally observed, that at a certain temperature the voltage dramatically increases with increasing temperature. This limits the applicability of liquid crystal media in the blue phase for display applications. A further disadvantage of the liquid crystal media described in the above cited documents is their moderate reliability which is insufficient for very demanding applications. This moderate reliability may be for example expressed in terms of the voltage holding ratio (VHR) parameter, which in liquid crystal media as described above may be below 90%.
Some compounds and compositions have been reported which possess a blue phase between the cholesteric phase and the isotropic phase that can usually be observed by optical microscopy. These compounds or compositions for which the blue phases are observed are typically single mesogenic compounds or mixtures showing a high chirality. However, generally the blue phases observed only extend over a very small temperature range, which is typically less than 1 degree centigrade wide, and/or the blue phase is located at rather inconvenient temperatures.
In order to operate the novel fast switching display mode of WO 2004/046 805 the light modulation medium to be used has to be in the blue phase over a broad range of temperatures encompassing ambient temperature, however. Thus, a light modulation medium possessing a blue phase, which is as wide as possible and conveniently located is required.
Therefore there is a strong need for a modulation medium with a blue phase with a wide phase range, which may be achieved either by an appropriate mixture of mesogenic compounds themselves or, preferably by mixing a host mixture with appropriate mesogenic properties with a single dopant or a mixture of dopants that stabilises the blue phase over a wide temperature range.
Summarizing, there is a need for liquid crystal media, which can be operated in liquid crystal displays, which are operated at temperatures where the media is in the blue phase, which provide the following technical improvements:                a reduced operating voltage,        a reduced temperature dependency of the operating voltage and        an improved reliability, e.g. VHR.        
It was an aim of the present invention to provide liquid crystal media suitable for use in blue mode or isotropic mode displays and provide one or more of the above-mentioned technical improvements. Another aim of the invention was to extend the pool of liquid crystal media available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that one or more of the above aims can be achieved by providing liquid crystal media as described and claimed hereinafter, which comprise one or more polymerisable mesogenic compounds or reactive mesogens (RMs) with a bent core.
Such compounds with a bent core, also known as “banana shaped” compounds, have been disclosed in prior art, for example in U.S. Pat. No. 6,596,193 as components of polymer dispersed LC displays. However, they have hitherto not been disclose or suggested for use in blue mode or isotropic mode displays.
Surprisingly, it has been found that the addition of such polymerisable mesogenic compounds or RMs with a bent core to mesogenic media exhibiting a blue phase results in the formation of a blue phase that has a wide process window >1.5° C. over which polymerisation can take place and a blue phase with a low switching hysteresis <2V.
Nematic liquid crystals doped with bent-shaped molecules having a blue phase have been reported by Z. Zheng et al., Journal of Physics 12 (2010), 113018.
The use of polymer stabilization to enhance the temperature range of a blue phase has been reported in T. Iwata et al., Mol. Cryst. Liq. Cryst. 470, 11-18.
WO 2006/061094 A1, WO 2006/069618 A1, WO 2006/079406 A1 and EP 1 816 180 disclose bent-shaped, non-reactive mesogenic compounds and their use in blue mode displays.
However, none of the above-cited documents discloses or suggests the use of bent-shaped polymerisable compounds or RMs in blue mode displays.